It is generally known, for instance from U.S. Pat. No. 3,350,734 dated Nov. 7, 1967 of Holm, to have a liquid solvent, such as trichloroethylene and tetrachloroethylene, kept at boiling temperature while maintaining a layer of vapor above the liquid phase by condensation of the vapor with cooling coils establishing a constant boundary layer between air and vapor, and it is also generally known to pass a strip of material into such liquid solvent, and/or into the vapor thereof, in order to dissolve grease present on the strip.
It is known to produce high energy ultrasonic agitation for precision cleaning of an intricate workpiece. The workpiece is immersed in a bath containing solvent through which ultrasonic energy is applied from a transducer mounted against the external wall of the bath. Intense and thorough cleaning of the most intricate surfaces of the workpiece is made possible by cavitation due to the formation, under energy at ultrasonic frequency, of gaseous bubbles within the liquid solvent which burst against the contaminated surface (see for instance U.S Pat. No. 3,066,688 of O'Neill dated Dec. 4, 1962.) It has been found that the alternate formation and collapse of bubbles in the solvent near the surface of the workpiece actually disintegrates particles of foreign matter thereon and result in an ultrasonic cleaning far more effective than any mechanical or chemical agent and without deterioration of the workpiece.
It is further known that a blanket of vapor above the surface of the solvent is necessary in order to prevent contamination of the solvent by infiltration of air, since the intensity of cavitation is reduced once air has been dissolved into the solvent. To this effect, an ultrasonic degreaser has been designed in which two solvent sumps are combined. One sump is at boiling temperature for generating vapor. A second sump is juxtaposed serving as a storage sump of distilled solvent. A common blanket of vapor is maintained above the surfaces of the two sumps by a condensating coil establishing a boundary with the open air at the top. The condensate from the second or storage sump overflows into the boiling sump. The workpiece is treated in the storage sump, with the transducer preferably mounted right under the tank.
Another difficulty exists with degreasers when the workpiece is a strip, or wire, instead of a single part. Then, continuous cleaning through the bath of solvent must be provided. It is known from U.S. Pat. No. 3,525,243 dated Aug. 25, 1970 to pass a wire through a bath of freon with transducers in the bath imparting longitudinal vibrations to the wire between two points thereof. It is also known from U.S. Pat. No. 3,582,400 dated June 1, 1971 to immerse cylindrical ultrasonic transducers in a bath of solvent and to unroll a wire through an axial opening of the transducer where it is exposed to cavitation agitation for cleaning purpose.
It is not desirable to immerse an ultrasonic transducer in a bath of solvent. First there is a risk of corrosion. Secondly, maintenance and replacement of the transducers are not facilitated by the environment of the fluid. Thirdly, as shown in the prior art, the choice of a transducer becomes more limited as a result of the requirement that it be placed into the bath. Finally, coupling of the continuous strip, or wire, in the bath with the transducer therein requires some special design to accommodate one with the other.
A preferred solution for effective cleaning of an elongated workpiece continuously supplied through a path of cavitation agitated fluid under ultrasonic energy is to pass the workpiece one way through the axis of a cylindrical transducer within a conduit mounted thereacross and to circulate fluid through this conduit continuously to and from two tanks separated by a cleaning stage so that clean fluid be continuously replenished at the interface between transducer and workpiece in the zone of cavitation. The constant supply of fresh solvent in the conduit along the axis of the cylindrical transducer insures a most effective cleaning action. However, maximum cleaning also requires total protection from the atmosphere.
The main object of the present invention is to provide ultrasonic cleaning apparatus for in line cleaning of continuous wire or strip in which the transducer is coupled to a cleaning solvent in liquid state without being itself in contact with the liquid solvent, and in which any interface of the solvent with air is prevented by a blanket of solvent vapor, but without exposing the transducer to vapor solvent.